This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which efficiently scan a volumetric region for three dimensional imaging.
When a volumetric region or three dimensional object is ultrasonically scanned for three dimensional imaging, it is desirable to completely and adequately sample or scan the region or object so that the resultant three dimensional image faithfully and completely represents the internal detail of the volumetric or three dimensional object. A number of techniques have been proposed for ultrasonically scanning volumetric regions with the array transducer scanheads widely in use today for conventional two dimensional planar imaging. One of these techniques is to rotate the scanhead about a pivot point. This technique will sweep the image plane through a cylindrical or conical volume of the body when the scanhead is rotated about the center of the image plane, depending upon whether the scan plane is linear or sector shaped. Both external and internally operating scanheads have been developed for performing this scanning. The article xe2x80x9cMultidimensional Ultrasonic Imaging for Cardiologyxe2x80x9d by McCann et al., published in the Proceedings of the IEEE, vol. 76, no. 9 (September 1988) at pages 1063-73 illustrates the rotational scan plane technique and describes an externally applied scanhead which scans the heart trans-thoracically. The scan plane is rotated by rotating a phased array transducer in angular increments with a stepper motor. The use of the motor enables uniform control of the angular rotational increments; in an illustrated application the scan plane is stepped in increments of exactly 1.8xc2x0. The rotational volumetric scanning technique can also be performed internal to the body with a multiplane transesophageal echocardiography (TEE) probe as described in U.S. Pat. No. 5,181,514. Since a multiplane TEE probe inherently performs the function of rotating an array transducer about its center, successive scan planes can be acquired and stored as the array transducer is rotated and then used to form a three dimensional image.
Another technique for radially scanning a volumetric region is with a two dimensional array transducer which electronically steers beams in different radial directions. With elements extending in two dimensions, the beams transmitted and received by a two dimensional array can be steered to electronically scan a conical or pyramidal volume by phased timing of the array elements. The use of a two dimensional array to scan a volumetric pyramid is shown in the article xe2x80x9cA Two-Dimensional Array for B-mode and Volumetric Imaging with Multiplexed Electrostrictive Elements, by R. E. Davidsen et al., Ultrasonic Imaging, vol 19 at pp 235-250 (1997). A complete ultrasound system for electronically scanning a volume with radially steered beams is shown in U.S. patent application Ser. No. 09/919,681, entitled xe2x80x9cThree Dimensional Ultrasonic Diagnostic Imaging With High Density Hexagonal Acquisitionxe2x80x9d by Cooley et al.
An improved rotational scanning technique for a volumetric region is described in U.S. patent application Ser. No. 09/433,124 by Lennon. Lennon has taken cognizance of the fact that the scanned beams are more closely spaced at the center of the volumetric region, and more widely spaced at the lateral periphery of the region. Lennon describes how to more uniformly spatially sample the volume by spreading the beams in the center of the volume and more closely spacing the beams at the periphery. This technique reduces the oversampling of the volumetric region at the center and the undersampling of the region at its periphery as the scanning plane is rotated through the volume.
The Lennon technique, while dealing with lateral spatial sampling variation, does not address another problem encountered in radial volumetric scanning, which is the axial spatial sampling variation. In the axial or depth dimension, conventional radial scanning results in a greater sampling density at and near the apex of the volume next to the transducer than is the case at deeper depths of the volume. This is true in both mechanical scanning with a one dimensional array and electronic scanning with a two dimensional array. It is desirable to be able to reduce or eliminate this axial sampling disparity in radial volumetric scanning. It is further desirable to improve the efficiency of the imaging system when doing so.
In accordance with the principles of the present invention, a technique and apparatus are provided for overcoming the disparity in sampling density between the near field and the far field when radially scanning a planar or volumetric region. This is accomplished by processing only partial raylines which have near field portions eliminated. The near field portions of some of the raylines can be discarded, thereby conserving image storage space and easing image processing demands. Alternatively, the partial raylines are only acquired in the far field. The partial raylines can also be formed by multiline acquisition or lateral interpolation which operates in the far field.